Wireless solutions

Choosing an RF communication protocol for your low-power wireless system

With the ability to connect up to 65,000 nodes in a network, the ZigBee® protocol is naturally an attractive choice for multi node low-power wireless systems—battery-operated systems that communicate over a short or long distance via a radio frequency. But there are alternatives.

This article tries to explain how IEEE 802.15.4, ZigBee and other protocols relate to each other.

IEEE 802.15.4

Several data communication standards are based on IEEE 802.15.4, so a brief explanation of it might make things easier to understand. This standard for wireless personal area networks uses radio-frequency (RF) for communication. It is intended for applications that have low requirements on data rate but high requirements on battery life and security. There is a wide range of products and application areas that fit into this category—safety, security and control of homes and commercial buildings, mobile services within business and healthcare, industrial automation, smart metering, home entertainment, asset management and much more.

IEEE 802.15.4 specifies the lower protocol layers: the physical layer (PHY) and the media access control (MAC). The physical layer provides the data transmission service by managing the Physical RF transceiver. This can work in one of three open unlicensed frequency bands:

the European 868.0-868.6 MHz: One communication channel

or North America 902-928 MHz: Up to thirty channel

Worldwide 2400-2483.5 MHz: Up to sixteen channels

The main function of the MAC is to handle the data service to the PHY, but also to handle several services such as validation, time slot handling and the management interface.

The standard also specifies two different setups on how the network can be constructed, peer-to-peer and star network. There are two different node types:

Full-function device (FFD) that can serve as the coordinator of a network as well as a common node

Reduced-function devices (RFD). These are meant to be extremely simple devices with very modest resource and communication requirements, only capable to communicate with FFDs. This also makes them cheap and low power.

ZigBee and other standards use the IEEE 802.15.4 as a base on top of which further layers are specified.

ZigBee

ZigBee specifies a high-level protocol for wireless communication. One of the first points addressed was the need for an automated ad-hoc construction of a network. Apart from the two methods of connection specified in IEEE 802.15.4, peer-to-peer and star network, the mesh network was added.

There are three types of nodes:

ZigBee Coordinator. May bridge different networks, one in each network

ZigBee Router. Can also run an application function.

ZigBee End Device. Can only talk to its parent. Low Power.

The ZigBee standard is proprietary to the ZigBee Alliance, an association of chip manufacturers, tool vendors and adopters of the standard. In order to secure different devices operate to work together, different profiles are decided, and also compliance testing is formalized.

6LoWPLAN

Just like the ZigBee standard, 6LoWPAN is intended for low-power RF-based applications but with a need for wireless Internet connectivity. As its name indicates, 6LoWPAN is a low-power wireless personal area network that is based on IPv6, the latest Internet protocol.

6LoWPAN allows IPv6 packets to be sent to and received using the IEEE 802.15.4 PHY and MAC. The range of application areas is vast—wireless sensor networks for home control, building automation, industrial automation and control, smart metering, health and medical monitoring just to mention a few.

Bluetooth and Bluetooth low energy

Unlike ZigBee and 6LoWPAN, Bluetooth is not based on IEEE 802.15.4. Bluetooth technology covers the PHY/MAC layers as well as high-level protocols and transmits short distance utilizing 79 channels at the 2.4 GHz band worldwide. Network construction is based on so called scatternets, consisting of two or more piconets. In a piconet, a master device can communicate with up to seven devices.

Bluetooth can be used to transmit virtually any type of data, including multimedia, but the power consumption has been significantly higher than that needed for transmissions using for example ZigBee. Has been? Yes, this has been addressed recently by the subset Bluetooth low energy (also known as Bluetooth Smart), which should put Bluetooth on the list when selecting which wireless protocol to use in your low-power wireless system.

Bluetooth is not standardized under IEEE (although it used to be) but instead managed by the non-profit Bluetooth Special Interest Group (SIG), which oversees development and licensing of the standard.

Homemade RF protocols

If your RF application is small and simple, you might want to consider writing your own RF protocol using a RF device working in one of the free frequency bands. This gives you full flexibility, but also quite a lot of work…